Well, it is definitely easier (esp. for parents/grandparents/etc) to buy as a gift, in disc form. And it will let you skip like 4GB(?) of the download.
And no, you won't need to keep the disc in the drive. Even if it installs of that, it still registers in your Steam account and runs like any other Steam program.
(I ended up going with the digital version, when every other Day1/AAA game I've gotten has been on disc. But that's only because I wanted to take advantage of pre-loading before the release day, so that I could actually play it that day. If I were buying it now, I'd get the disc.)
Someone in another thread was complaining that the entire game is not on the disc, only a small portion of it. The majority of the game still has to be downloaded.